CORE SUMMARY / ABSTRACT The Immunopathology Core is a central resource for all three projects in the PPG. The Specific Aims are to provide: 1) expert pathological and immunopathological interpretation of the allograft in each of the three projects, which is critical in the evaluation of tolerance or rejection; 2) innovative staining methods directed at answering the specific scientific questions that arise in each of the projects as requested by the investigators; 3) high quality, uniform histochemical/immunohistochemical preparations common to all projects. 4) advice on the design of experiments to optimize the potential pathological studies; 5) ready investigator access to scanned images and digital photographs for publication; and 6) common and advanced computer analysis for morphometrics and 3-D reconstruction. Each project will use the Core to determine the status of the allograft along the axis of tolerance--rejection, and the specific features of either (e.g., are there intragraft Treg-rich organized lymphoid structures (TOLS) or is the graft free of infiltrate; is the rejection antibody or T cell mediated?). The techniques to be provided include: standard scoring of allograft samples (Banff, ISHLT and custom schema), immunohistochemistry and immunofluorescence for cell identification and status, whole slide digital imaging and morphometrics for quantitation of infiltrates and vascular lesions. Each project will use the Core to determine the status of the allograft along the axis of tolerance--rejection, and the specific intragraft features of either. The results from the Core analysis will be essential to assess the success and mechanisms of the protocols.